It is known design practice in the automotive power transmission art to provide a main drive clutch assembly for establishing and interrupting a torque flow path between an engine and torque input elements of multiple ratio gearing. The clutch is required to establish controlled torque transfer during start-up of the vehicle as well as to provide torque delivery interruption during ratio shifts of the multiple ratio gearing. Although friction clutches used in such an environment typically are actuated and released by a driver-operated clutch linkage mechanism, it is known practice in the case of semi-automatic drivelines to effect gear ratio changing functions and clutch release and engaging functions by remote clutch actuators without a direct mechanical linkage to operator controls.
Known prior art designs in which such remote actuators are used include electromagnetically actuated ball ramp actuator mechanisms that act in conjunction with friction discs of a transmission neutral clutch or main clutch to provide a clamping force against friction discs of the clutch. An example of a transmission clutch of this type may be seen by referring to U.S. Pat. Nos. 5,584,776 and 5,485,894, which are assigned to the assignee of the present invention. A fluid, pressure-operated servo actuator for actuating the ball ramp actuator mechanism for engaging a friction clutch may be seen by referring to U.S. Pat. No. 3,144,107, which discloses a ball ramp actuator mechanism for a steering clutch for a tracked vehicle. In each of these prior art designs, the ball ramp actuator mechanism consists of driving and driven side members with opposed ramp recesses and load transferring balls in the recesses. Relative angular motion of one side member with respect to the other will establish a clutch engaging force on a friction clutch assembly as the side members shift axially, one with respect to the other. In the case of ball ramp actuator mechanisms that are electromagnetically actuated, the electromagnetic coil of the actuator must be energized throughout the clutch engagement time.
Prior art clutch constructions using electromagnetically actuated ball and ramp mechanisms typically are unidirectional in their torque transfer function. If the relative rotation of the side members of the ball ramp mechanism should reverse, the clutch engaging force would be interrupted. If a clutch actuator mechanism of this type were to be used in an automotive vehicle driveline, for example, the torque reversal that would accompany a coast condition of the vehicle would result in disengagement of the friction clutch so that engine braking during coasting could not be achieved.
An example of an electromagnetically actuated ball ramp actuator mechanism for a friction clutch that incorporates a coast braking feature may be seen by referring to U.S. Pat. No. 5,469,948. That design, however, requires a relatively complex overrunning coupling and an auxiliary clutch disc whereby the overrunning coupling provides a one-way driving torque connection between one side member of a ball ramp mechanism and a disc situated between the opposing side member of the mechanism and the electromagnetic actuator. The auxiliary disc thus becomes coupled through the electromagnetic interface between the ball ramp mechanism and the electromagnetic actuator so that torque can be distributed in a reverse direction through the friction clutch mechanism associated with the ball ramp actuator.